(a) Field of the Invention
The present invention relates generally to a polarization independent optical attenuator. More particularly, it relates to an improved optical attenuator having a tandem arrangement of three birefringent wedges and a Faraday rotator.
(b) Description of Related Art
Optical attenuators or isolators are generally used to prevent the reflected portions of an optical transmission from re-entering the transmitting device. Optical attenuation may be used to improve the quality of an optical beam, or to prevent damage to the transmitting device. Solid state laser systems, for example, depend on optical attenuation to improve beam quality and to prevent reflected output from seriously damaging the elements generating the laser output.
A variety of optical attenuators comprising a non-reciprocal magneto-optic conduit (such as a Faraday rotator) and polarizers are well-known to those skilled in the art. A Faraday rotator is made from an optically transmissive, magnetically activated conduit. A Faraday rotator imparts a non-reciprocal (absolute) rotation to the polarization vector of an optical or lightwave signal traveling through it. Thus, a lightwave's polarization vector will be rotated in the same direction (i.e. clockwise or counterclockwise) regardless of whether it passes through the Faraday rotator in a forward or a reverse direction. The amount of rotation is a function of the couduit material and geometry, and the intensity and orientation of the applied magnetic field.
A known optical attenuator configuration illustrated in FIG. 1 uses a tandem arrangement of dichroic polarizers and a Faraday Rotator. Linearly polarized light (represented by a solid line) travels in a +z, or forward direction through a dichroic polarizer P1. The changing polarization state of the forward traveling light is represented by a series of arrows surrounded with a solid circle. Ideally, the polarization vector of the incident lightwave is aligned with the polarization direction of polarizer P1 to minimize source attenuation. The lightwave continues through a Faraday rotator FR wherein its polarization vector is rotated 45.degree. clockwise. A second dichroic polarizer P2 is ideally arranged so that its polarization direction matches that of the lightwave after it has traveled through the Faraday rotator. The lightwave then passes through polarizer P2 substantially unattenuated.
A reflected lightwave (represented by a dashed line) traveling in a -z, or reverse direction will easily pass through polarizer P2 because its polarization is substantially identical to that of the lightwave exiting P2. The changing polarization state of the reverse traveling light is represented by a series of arrows surrounded with a dashed circle. Due to the non-reciprocal nature of the Faraday rotator, the lightwave traveling in the reverse direction will have its polarization direction rotated 45.degree. clockwise. As a result, the lightwave traveling in the reverse direction is completely blocked by P1 because its polarization vector is perpendicular to the polarization direction of P1. Thus, the optical isolator configuration of FIG. 1 produces optical isolation by precisely aligning the polarization direction of reflected lightwaves so that they are substantially absorbed (attenuated) by a dichroic polarizer.
The simple optical attenuator configuration of FIG. 1 has several problems. First, incident lightwaves that have a non-linear or arbitrary polarization direction will be significantly attenuated by polarizer P1. Second, high attenuation or isolation of reflected lightwaves is extremely difficult to achieve because accurate rotation of the lightwave's polarization vector requires precise control over a multitude of variables.
The second problem relates to the rotation angle imparted by the Faraday Rotator. The rotation angle that a Faraday rotator imparts to a lightwave traveling through it depends on the material and geometry of the conduit, the intensity and orientation of the applied magnetic field, the conduit's temperature, and the wavelength of the lightwave. Small changes in any of these parameters can cause the lightwave's polarization vector rotation to be more or less than 45.degree.. Deviations from an ideal 45.degree. rotation will result in some tranmission of the reflected lightwave through P1 because its polarization vector will have a non-orthogonal orientation to P1's polarization direction. Isolation performance can be improved by adjusting P1 so that its polarization direction is orthogonal to the reflected lightwave's polarization vector. However, even a linearly polarized source would then be substantially attenuated by P1 as it passed through in the forward direction because the polarization of P1 would be non-orthogonal with respect to it.
Another known attenuator configuration shown in FIG. 2a achieves polarization insensitive isolation using a tandem arrangement of birefringent crystals (P1 and P2), a reciprocal rotator HWP, and a Faraday rotator FR. The birefringent crystals have two polarization dependent indices of refraction. A lightwave traveling through the birefringent crystal splits into two spatially divergent rays (i.e. an ordinary ray and an extraordinary ray) having orthogonal polarization directions. In this way, an arbitrarily polarized lightwave can be entirely resolved into a pair of spatially divergent, orthogonally polarized rays that pass through the crystal substantially unattenuated.
A lightwave traveling in the +z or forward direction enters birefringent crystal P1 and splits into two rays having orthogonal polarization directions. An ordinary ray travels straight through the crystal while an extraordinary ray follows an oblique propagation path through the crystal. Both rays exit the crystal P1 and travel in parallel through the Faraday rotator wherein their polarization vectors are rotated clockwise 45.degree.. The rays next pass through the reciprocal rotator wherein their polarization vectors are rotated clockwise an additional 45.degree.. The polarization directions for both rays have then rotated a total of 90.degree. clockwise. Since crystal P2 is substantially identical to P1, the ordinary ray's polarization direction forces it to follow the oblique propagation path through the crystal P2 and the extraordinary ray now follows a straight path through P2. As a result, the rays converge and recompose the original lightwave as they exit crystal P2.
As shown in FIG. 2b, a reflected lightwave traveling in a -z or reverse direction will retrace the path of the forward traveling ray up through the Faraday rotator. At the output of the Faraday rotator, though, the rays will have polarization directions rotated 90.degree. with respect to those of the forward traveling rays. Consequently, as the rays traveling in the reverse direction pass through P1 they will diverge and thereby avoid the aperture of the optical transmission device. In sum, the optical isolator of FIG. 2a produces optical isolation by causing reflected lightwaves to diverge spatially so that they avoid the aperture of an optical transmission device.
Polarization insensitive optical attenuators similar to that shown in FIG. 2a are not suitable for use with large diameter optical beams (e.g. high-powered laser systems). With the configuration shown in FIG. 2a, the birefringent crystals have a parallel plate geometry. For this geometry, the spacial separation of the two rays is primarily a function of the plate thickness. As a result, an optical beam having a diameter greater than approximately 3 mm cannot be sufficiently separated into two distinct rays given the practical limitations on plate thickness.
Other known optical attenuators have modified the geometry of the birefringent crystals to be that of a wedge shape. One example of such an attenuator is shown in FIG. 2c and FIG. 2d. A wedge geometry allows for a greater degree of spacial separation between the ordinary and extraordinary rays. By properly orientating the optical axis of a birefringent wedge the orthogonal components travel at different rates along parallel propagation paths through the body of the crystal. As the orthogonal components exit a beveled surface of a wedge shaped crystal they diverge at different angles because the indices of refraction for the two polarization directions are different. Thus, greater spacial separation can be achieved between the two rays by increasing the path length through the optical attenuator.
The above approaches using birefringent wedges cannot be easily integrated into systems having large or high-powered optical beams because the spatially separated orthogonal rays must be recombined using a focusing lens that cannot be readily incorporated into apertures of high powered optical equipment such as laser amplifiers. Accordingly, there is a need for an improved polarization insensitive optical isolator that can be easily adapted for use with high-powered optical systems.